1. Field of the Invention
The present invention relates to a filter medium, a fluid filter, and an engine oil filter. More particularly, the present invention relates to a filter medium that exhibits high filtering efficiency and has a long filter life, as well as a fluid filter and an engine oil filter including this filter medium.
2. Description of the Related Art
Various filters are used conventionally to remove microparticles and the like from a fluid. For example, lubricating oil used in an internal combustion engine contains suspended sludge, dust particles, metal dust, carbon particles generated as a result of incomplete combustion, and so on. When these particles exist, problems such as an increase in the viscosity of the lubricating oil, defective lubrication of the engine, and a reduction in the life of the lubricating oil arise. To remove these particles from the lubricating oil, an oil filter is used. In addition, an air filter, a liquid filter (for example, a fuel filter or a hydraulic filter for large-scale civil engineering/construction machines and the like) are also used. An air filter is capable of trapping dust particles in the air. A liquid filter is capable of removing particles contained in a liquid with a high degree of efficiency so that a pure liquid can be obtained.
Resin-treated filter paper, nonwoven fabric, felt, and so on are typically employed as filter media for use in a filter. For example, JP-A-H9-841 discloses a filter medium constituted by fibrillated organic fibers, extremely fine organic fibers, crimped fibers or fibers having a modified cross-section and a specific maximum projection diameter, and a fibrous organic binder or a liquid binder. In this filter medium, the extremely fine organic fibers construct networks between the fibrillated organic fibers. As a result of this network construction, the filter medium described in JP-A-H9-841 achieves an increase in the fineness of the filter pores, thereby improving the filtering efficiency.
Exhaust gas recirculation (EGR) systems which extract a part of the exhaust gas from an exhaust system and recirculate it to an intake system are being introduced into automobile engines. In an engine installed with an EGR system, the amount of microparticles such as carbon sludge contained in the lubricating oil is presumed to increase as the EGR ratio (EGR amount/intake air amount) increases. Furthermore, due to improvements in lubricating oil, the microparticles are more easily dispersed, and therefore the carbon sludge contained in the lubricating oil is showing a tendency to increase in fineness. For these reasons, there is demand for an automobile engine filter and a filter medium that can trap carbon sludge and other microparticles more reliably. Moreover, the demand for a filter and a filter medium exhibiting superior filtering efficiency is not limited to the automotive field, and extends to various other technical fields.
Reducing the pore diameter of a filter medium has been proposed in the related art as a method of improving the filtering efficiency of the filter medium. However, when the pore diameter of the filter medium is reduced, the filter life is shortened, leading to an increase in the frequency with which the filter must be replaced. Hence, there is demand for a filter medium that exhibits superior filtering efficiency and also has a long filter life.
In the filter medium described in JP-A-H9-841, aromatic polyamide fiber is preferably employed as the organic fiber. However, aromatic polyamide fiber has a large specific gravity and exhibits poor hydrophilicity. A filter medium is typically formed by making a slurry containing fibers into paper using a net. The poorly hydrophilic fibers and heavy fibers immediately accumulate in the slurry. Hence, in a case where aromatic polyamide fibers are used, the fibers accumulate during manufacture of the filter without sufficient voids between the fibers, and therefore the voids required for the extremely fine organic fibers to disperse cannot be secured sufficiently. As a result, unless a large amount of extremely fine organic fibers is introduced, networks are not formed among the extremely fine organic fibers, and therefore sufficient filtering efficiency and a sufficiently long filter life cannot be achieved. On the other hand, when a large amount of extremely fine organic fibers is introduced, the strength of the filter medium decreases, which is highly problematic in terms of practical application. Moreover, aromatic polyamide fiber is not sufficiently hydrophilic, as noted above, and therefore does not disperse favorably in water. As a result, the extremely fine organic fibers cannot be dispersed sufficiently, and networks cannot be formed among the extremely fine organic fibers. Due to these factors, it is difficult to manufacture a filter medium having a bulky structure using the fiber compositions of the related art.